Aaron R. Ybarra, 26, was booked into the King County Jail late Thursday for investigation of murder, police said.

At an earlier news conference, Assistant Police Chief Paul McDonagh said the man in custody was not a student at the school.

The afternoon shooting came a week before the end of the school year, and the situation was particularly tense when police initially reported that they were searching for a second suspect.

“It appears the suspect acted alone,” McDonagh said. Initially, “there was a report of another person with a weapon, but (investigators) have not confirmed that.”

He said he did not know the gunman’s motive or intended target. Detectives are “working as quickly as we can to figure it out,” McDonagh said.

Messages left with friends and relatives of Ybarra via social media were not immediately returned.

The Seattle Times said the suspect’s father, Ambrose Ybarra, said he doesn’t know anything of the incident.

“We just hope he’s safe,” he told the paper. “It’s upsetting to have these accusations thrown around. We’re in emergency mode. We are trying to stay calm.”

The paper said Zack McKinley described himself as one of Ybarra’s closest friends and said he was “super happy and friendly.”

McKinley said the attack was puzzling because Ybarra had happy to have just started a new job bagging groceries at a store.

He said Ybarra didn’t do drugs or drink alcohol and spent time writing. Ybarra could get emotionally low, but McKinley said he had a good group of friends and never saw him depressed.

Also late Thursday, police who said they were serving a warrant entered a house that was believed tied to Ybarra. A phone message left at that house in a north Seattle suburb was not immediately returned.

The university locked down its campus for several hours, and it alerted students and staff to stay inside. Some students were taking finals in the same building that the shooter entered.

A 19-year-old man died at Harborview Medical Center. A hospital spokeswoman says a 20-year-old woman remains in critical condition after hours of surgery. Another man is hospitalized in satisfactory condition. A fourth person was treated and released.